The Cartridge/Headshell join or an Introduction to Sound Propagation and Possible Solutions - an interview with Pierre Lurné

Introduction

For audiophiles Pierre Lurné needs little introduction, designer for Goldmund, owner and designer of Audiomeca and author of many papers on the subject of hifi - especially when related to vinyl reproduction. Here we have an interview we did last year and which I've finally got round to editing and HTMLing - it's illustrative and more important it contains a tip that any of us can try - I know I intend to :-) I apologise in advance for any errors I may have introduced into the text, though I have tried to retain Pierre's 'voice' - just read it with a French accent :-)

Pierre - much of your work has been in the field of mass distribution and the like on tonearms, but the propogation of vibrations is another 'hobby hourse of yours, it doesn't seem to be something much talked about?

In HiFi like in everything else, there are subjects in fashion, and forgotten subjects. Some are overexposed and others never mentioned. The importance they have in the final result has nothing to do with it. Thus, ridiculous topics are regularly commented on and first class fundamental principles ignored. In the worse cases Physics is ignores to be replaced with biased questions, dishonesty, idleness, magic and moreover and we end up with a real mess where musical quality depends on the luck of the draw. A paradox in a rather scientific field. Not 100% scientific we agree, though “Good Physics = Good Results” is almost always the rule. “Almost” as Hifi is not an exact science (happily) ! Anyway, “The Exception Makes the Rule”.

Here is one of these tricky subjects. One among many : The Cartridge/Headshell join proves to be a very underestimated one despite its critical place so close to the tracking hot spot.

In most arms the conventional and usual join, headshell and armtube screwed together, is not bad enough to prevent good musical results that we all have experienced. In comparison, Mass Distribution and Tube Rigidity for instance are major parameter. But anyhow, fortunately the Phonographic Process is so tolerant/indulgent that it nearly always works ! Those rudimentary phonographs from the past did work ! Those awful cheap record players for children do work ! They just ‘sound’ like they are - awful. On the other hand, we all know good equipment makes good music in spite of many remaining drawbacks. BUT They just do not sound as right as they could do. Waiting for better adjustments or improvements or progress. This is both the fortune and the misfortune of the art of Phonography in the same time. In some ways life would be a lot simpler if design mistakes would lead to music destruction; noises and cacophony, then the mistakes would be more obvious, lots of products would not survive or even be launched, but Phonography is a good boy and always works and there are always some people able to sell anything to credulous customers.

BUT (again) poor design does not lead to Excellence. Next to the cartridge, right from the start, at the SOURCE point, the join with the arm headshell acts as one of the very first limitations of the whole reproduction chain. Lost at the Source, lost forever.

So far, the Cartridge/headshell join does not comply with the hard principles of Sound Propagation and requires better understanding and improvement.

As a design this join has a major effect on the Acoustic Line, Acoustic Impedance, Hard path, Hard point, Mechanical Grounding, Signal Distortion, Signal Conduction, Signal expansion, Delays, decays, all critical for Vibration Control and all weak. Even Mass Distribution is involved as it helps good Vibration Control, but as it is sadly never respected… The join is so because of a very old Phonography Principle; because the cartridge body is not made one piece solid with the Arm wand becaue we want the right to chose our favorite cartridge and to change it whenever we want; because ultimately we want a domestically acceptable and commercial product.

Sound or Vibration Propagation is an extremely complex branch of Physics which requires some of the biggest computers of the world. As yet Sound Propagation is not one of the Hifi designer’s tools nor is it obvious at an intuitive level. Spikes and Mechanical Grounding being very rudimentary examples. The theory explains that a vibration, emitted from a source point, “propagates” and follows the path of least resistance. Inside an homogeneous body (solid, liquid or gaz), a simple vibration spreads in an ever growing spherical wave regularly decaying till complete extinction in a similar way as the circles created by a stone thrown on a water surface. In a limited body, when the vibration wave touches the border, it is spit in two components. A part goes through and keeps on its way into the next body and the other part reflects and gets back in the direction of its origin point. How much goes through and how much is reflected depends on the “Acoustic Impedance” of the materials, on the pressure, on the surfaces qualities of the join and other parameters. The speed of the vibration depends on the material being crossed. As a general rule, the harder the material the higher the speed.

In a hard and small body, the reflected wave, still in Expansion Mode, comes back to its origin – Remember that ! - goes through it, keeps on its way, reaches the opposite border, gets split and reflected again, crosses again its origin and so on and on till complete natural decay and extinction. From this elementary figure, one can easily imagine how much things get quickly extremely complicated when the vibrations are numerous, continuous, and ever changing (in frequency, amplitude, transient etc…) like a musical signal. Extremely complicated even more when the body has various shapes, when it is hollow, when there are holes, when it is in contact with other bodies made from identical or different materials, when it is under stress (screw pressure), when the surface is rough or polished, covered by paint or metal plated etc… Extremely complicated again when resonances set up due to extra devices and/or particular geometric shapes caused by design mistakes. Resonances always set up for they have infinite possibilities to occur. You are just starting to understand why Sound Propagation requires some of the biggest computers in Physics research.

Vibrations literally invade everything, they bounce everywhere, they are split a number of times, they speed up, they slow, they flow better on the external surfaces (Known as Skin Effect), they increase under particular conditions (Resonances, reverberations, echoes, distortions…) and they quickly reduce and die if…IF, and only IF, the original excitation stops. As long as the initial excitation continues, they establish inevitably. An absolutely Natural Phenomenon.

And you have to imagine the sort of accuracy we need with vinyl reproduction

(Including extracts from Galen Carol website Texas with his kind permission).

SIZE : when one stops to ponder the relationship of stylus to groove, the numbers boggle the mind. Groove width on a vinyl LP ranges from 0.025 mm to 0.035 mm (let’s say 3/100th mm). Contrast that with a human hair which measures 0.17 to 0.20 mm.

PRESSURE : though a tracking force of 2 grams seems rather insignificant, when translated to the small area of the stylus contact, we are looking at an incredibly high pressure. On a non-spinning record, the stylus stresses the groove till permanent deformation. Better to avoid this!. In usual Spinning Mode, the groove is safe. The Elasticity of the vinyl material is a well known character from mathematicians.

ACCELERATION : Much worse in the dynamic domain. The groove has to withstand such incredibly high accelerations that are seldom seen in other fields of Physics.

It is amazing that vinyl reproduction works at all, much less as well as it does...

Even more unbelievable is the size of the engraved low-level information. Depending on the loudness at which the system is being played, you can usually hear down about -60 dB, which means you are hearing groove displacements of the order of a few millionth! That’s like splitting a hair thickness into one thousand pieces.

You understand now why and how those vibrations mentioned above are detrimental to the tracking quality and ultimately to the music. At this scale NOTHING is insignificant.

We have quickly seen above that bouncing all around, vibration waves return to their origin again and again. That is our point! When the stylus is tracking a record, vibrations, the music itself, are emitted in all directions, including in the surrounding air which we directly hear if we're close enough. This is the so-called “Needle Talk”. But most of the vibrations propagate in the record and the turntable on one side and in the cartridge and the arm on the other side. All those nasty vibes come back to the stylus and are tracked AGAIN ! Again and again ! This natural phenomenon isalways at work and inevitably blurs the music.

Imagine - If the musical signal engraved in the record groove is tracked only once by the stylus, it is then immediately corrupted by the addition of hundreds distorted and variously delayed bad copies of itself. Fortunately the speed of the phenomenon is very fast and cannot be perceived with a sufficient delay to translate in a true echo. Instead, it could be compared to a small and short distorted reverberation. I Hope there's no need to point out the extra job they give to the stylus cannot be without consequence on tracking. No need as well to say that they span the all frequency range and excite all resonances around. One could argue there are bad vibes coming from elsewhere as well. Correct! Bad enough that other unwanted vibrations come from the outside, from the turntable motor for example and from the house, the refrigerator, the air conditioning, the footsteps, the cars and the trucks on the road nearby and more. But these cause much less harm than the internal ones. Why ? All being equal, random external vibes cannot be as detrimental because their harmonic content has absolutely nothing to do with the music being played. They are nothing but random noise and good turntables usually deal with such things quite well

Besides they arrive somewhat diluted and tired from more remote sources. Conversely; internal vibrations must be considered as the worse parasites around. Not only they are fresh and the music itself but they are generated from the very same point which will pick them up again and again... Indeed as a Natural Phenomenon, and it works on musical instruments too. They build their qualities character and harmonic richness. But this is quite different because the job of High Fidelity is totally opposite. The artistic event being duly complete and recorded, the reproduction/playback must respect it entirely with no additions or subtractions whatsoever – that is High Fidelity...

Mechanics proposes several well known solutions to the Analogue designers for vibration control : High Mass, Dynamic Balancing, choice of the materials, uses of lead, dissipation, draining, damping… Vibrations can be reduced, trapped, delayed, damped, split, conducted, evacuated, stopped… As for the majority of the potential resonances that can be fixed, damped, reduced, tuned etc… But the best mechanical solutions do little if we cannot understand what is going on exactly inside the materials. From the cartridge to the outside, thus mainly through and into the arm. The theoretical 'perfect' Mechanical Line would have to be made from only one hard material for vibration conduction, its forms always increasing, kind of tapering or horn to infinity for full rigidity, for total draining till natural extinction and without any feedback.

This is obviously impossible, but our own situation is far worse with our short pickup arm, and its various other unavoidable features such as the break needed for bearings to allow the two-degrees movement of the bearing to follow the groove displacement and the record warps, or the Mass restriction, the multi-part construction etc… Here we inevitably reach some of the limitations and compromises inherent to the Phonographic process.

Yet if Perfection does not exist on this world, the designer must approximate to it. His job, duty and fun ! Happily the Physics does not leave him short and there's much room for improvement. Each time a cut/a break occurs along the Mechanical Line, it adds to the phenomenon described above. Too many cuts quickly make the concerned device totally out of control and unpredictable/unforeseeable. Each part acts as a “Sound trap” instead of quickly funneling parasitic energy away. A long Mechanical Line made from several parts attached to each other multiplies the problems to a large extend and causes a “congestion” of the vibrations, similar to trapped pressure, which cannot find the way out. Things get worse “naturally” with over complicated forms and incorrect choice of the materials.

In an arm the Mechanical Line starts at the stylus and ends at the Arm bearing. As we have seen before, this line inevitably, and on principle, suffers from several breaks. We shall not discuss the particular case of the cartridge here (as there are many variations). We start from our cartridge to headshell joint, the first-break. Then, the next inevitable break will be the bearing. These two breaks define the simplist possible figure. No doubt that a removable headshell for instance has practical advantages but it adds to the price to pay. The SME standard, with its lock-ring, being the worst solution by far as it creates at least two breaks and badly compromises the tube rigidity. Definitely not an audiophile item.
Most of the time the strict tube begins at the headshell and is mounted at its opposite extremity to the bearing housing. Then we face to the various bearing kinds, from the over complex (in terms of vibration behavior) 4 ball-bearings 'gimballed' system to the simple unipivot solution. Here, it is worth noting that the unipivot's simplicity talks for itself, though the bearing used is but one arm parameter among many. In other words, the best bearing is not the guarantee for the best musical results.

So... The simplest “arm-tube Mechanical Line”, has 2 breaks. Some Arms make much more complicated lines with up to 10 breaks or more and with the predictable bad side-effects. I despair. A lost cause.

To summarize, Sound Propagation and therefore Vibration Control has to favor a one-piece headshell + tube + bearing housing assembly, with full attention to rigidity and damping devices.

If your aim is top musical reproduction then we must consider the Sound Propagation requirements and before convenience...

In the seventies, some used to say that the harder they tightened the cartridge on the headshell the better. Logical as they approximated the one piece solid figure. It would have been interesting recalling the little story of that audiophile group who worked on the opposite way for a while but instead let’s focus on the description of a nice hint, dating from the same period of time, which will be the perfect illustration of our subject :

The Usual Cartridge/Headshell joint is secured by a couple of little screws. This joint proves to be rather poor. The screw pressure is limited by the size of the screws and the risk of damage; the surface contact, typically 1 to 2 mm², is generally too weak - reject any nylon hardware here - the nature and finish of the surfaces matter as well. As a result :

-a- This joint being inevitable (see part 2), the one-piece solid-figure does not exist (Only one single case recorded). Consequently the other possibilities range from :

-b- Separated Cartridge and Arm headshell both made from the same material and flat surfaces of contact. Very common and not so bad.

-c- Plastic cartridge body, metallic headshell and random surfaces. Very common as well and very bad.

Aha! The promised 'Tip'! The One Single Hard Point. - Not commercially available but already encountered elsewhere. Spikes belongs to this category. The single point of contact imposes “One way out and one only”; the contact pressure being high, parasite energy flows through easily and hardly comes back; the geometrical stability of the critical cartridge to bearing axis is complete. Besides and as we are going to see, there are extra provisions for interesting damping, VTA and Azimut tunings.

Caution: this hint is intended to the knowledgeable audiophile who is familiar with technical things. Remember a cartridge is very fragile and very expensive!

Look at your Cartridge/Headshell mounting. Locate their surface contact. No need to disconnect the mini pins. Unscrew your cartridge of 3 to 4 turns.

From now on your precious minute adjustments are all lost and will have to be redone. This operation clears a tiny gap between cartridge and headshell. Prepare a paper strip, around 9X40mm, roll it up and flatten it out. Cutting parts adjusts the thickness. Insert this little wedge in the gap through the cartridge front and between the 2 screws. Secure the position tightening the screws gently only. Prepare a tiny bit of a sewing needle* – just a couple of mm long. Insert it in the gap at the cartridge back. On the middle of the back. That’s it ! The joint is now restricted to the needle contact. The paper wedge acts as a damper right at the critical place. Depending on the screws pressure you get extra ways to tuned VTA and Azimuth. More or less strongly tighten the screws, in all cases the pressure is massive because of the tiny contact point of the needle. Unlike the normal situation this means that even plastic /nylon screws can be considered. Perform again the usual cartridge adjustments. Arm height too. A tricky job - sorry. A job to be checked and fine tuned several times in the future.

Enjoy ! Yes it is not perfect, nothing is perfect on this earth, but it is much better! From that basic principle one can imagine many variations. Once again take care with your cartridge stylus. Break nothing and never turn Old Good Physics upside down.

(Caution : this hint cannot be used on some designs).

Conclusions

Currently Mass Distribution and Tube Rigidity for instance, are seen as absolute prerequisites to high music quality, Sound Propagation requirements are generally ignored.

All parameters are not equally important but attention to Sound Propagation must be better considered as the Final Touch. Maybe it's the difference between good and excellent, this will remain up to you.

The applications of the principles of Sound Propagation in the Hifi field have failed to seriously develop so far. They are still in the infancy and for that reason the author asks for indulgence. As a general rule it is correct, though many details must be further studied and confirmed over a long period of time. Today it is ironic to see in the same time the demise of the CD disc, in favor of ‘all computer music’, and yet we have the incredible resistance of the vinyl record which is still with us and improving almost 30 years after the notification of its death. It will probably be alive still in 30 years time – let's hope so :-)

Pierre Lurné

(*) Geoff's note :-) - having looked at this, it appears that the same effect could also be achieved by using a very small ball bearing. In my case I embedded this into a small blob of Blu-tac which I then stuck on the underneath of the headshell in the correct spot before then refitting the cartridge. For me this was less fiddle and seemed to work OK...

© Copyright 2011 Geoff Husband - geoff@tnt-audio.com - www.tnt-audio.com